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(CNN)If only Joe Alvarado had intervened when 15-year-old Moriah Gonzales was killed just a few feet away from him.

Then maybe she would still be alive.

"It is what it is. Everybody dies," Alvarado told CNN's "The Hunt with John Walsh."

"I guess somewhere inside me there's a regret of not doing something. I could've done something. I know that I could've."

If only he had.

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But he didn't.

Alvarado is serving a 34-year sentence for murder in a Texas prison, where he revealed shocking details alleging how his ex-girlfriend Jennifer Puente killed Moriah while he sat just a few feet away and how he said he helped Puente burn Moriah's body.

Growing up south of Dallas in Waxahachie, Texas, Moriah was a fun-loving, headstrong teen with an adult attitude.

"She would pick a fight with anybody that started anything with anybody that she loved," remembered Kendall Dodson, her best friend.

The youngest of four sisters in a tight-knit family, Moriah was blessed with a doting mother and was the apple of her father's eye. Robert Gonzales is a former boxer who repeatedly denied his daughter's requests to teach her how to fight.

He would always say no because he wanted her to grow up first, he said.

Moriah grew up fast when she started hanging with an older crowd, including 18-year-old Jennifer Puente. Police described Puente as an "alpha female" — a "tough, young girl used to getting her way." Puente wore her defiance permanently inked on her body.

"The [tattoo] that stood out to me was the one that said, 'F--- everybody' on her shoulder," said Texas Ranger Jason Bobo.

Jennifer had a baby boy with a man named Johnny Wells Jr. When Wells launched a custody battle for his son, police said Jennifer showed her violent nature by slashing the tires on Wells' vehicle.

"Jennifer was making threats, wanting to kill people, burn bodies and so forth. And I was thinking in my head, I mean this stuff was serious," Wells said.

The murder

Alvarado first met Puente at a plastic housewares factory in Ennis, where they both worked.

They'd only known each other for two or three weeks before the day of the murder. It was Friday, March 16, 2012. Alvarado said he, Puente and Moriah were hanging out at his mother's house smoking a synthetic form of marijuana called K2.

"Jennifer was doing some cocaine," he said. Then she put a knife under Moriah's chin, he said, and asked Moriah, "'What is it that you said about not being afraid to die?'"

"Moriah told her she wasn't afraid," he said. Moriah told Puente she didn't have the guts to do it, he said, but apparently Puente followed through on her threat after a heated struggle, stabbing Moriah in the abdomen.

"Moriah was telling her, 'You're f---ing crazy, get off me.' I was under the influence. I really didn't want to be involved."

The room grew quiet as Moriah lay motionless with Puente on top of her, Alvarado said.

"What the hell?" Alvarado asked. "What'd you do? Why'd you do it?"

For the sense of power was Puente's reply, according to her former boyfriend. "Jennifer had carved a pentagram and some other stuff on Moriah's body," he said.

Alvarado and Puente agreed to dump the body, he said, deciding to leave Moriah along side a road that the couple took to get to work. "I instructed Jennifer how to pour the gas," he said. "She did it pretty carelessly. Then we set fire to her."

At 6:59 a.m. on Saturday, March 17, a 911 caller reported what looked like a burning "human body" on the roadside.

On the run

After stopping to get gas and cigarettes, Alvarado said he and Puente "went back to the house and chilled for a little while." Alvarado said Jennifer "seemed pretty nonchalant" about the killing. "She didn't feel any remorse over it," he said. "She was proud of what she had done."

Jennifer continued to go to work, he said. "Me, I just was kind of numb from the experience. I just got high every day."

When they saw news reports about Moriah's body being found, Alvarado said, they decided to run.

After authorities ID'd the body as Moriah's, police pulled her phone records and learned that she had last called Puente. Puente's sister then told police that Puente had said she and Alvarado had killed Moriah.

An anonymous tip tracked Puente and Alvarado to a motel in Grand Prairie. Authorities picked up Alvarado nearby when an officer spotted him on the street.

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When police arrived at the motel room, Puente was gone. The room was a mess, strewn with fast food sacks and empty beer bottles.

Maybe because she had fled the room quickly, police said, Puente had left her purse and wallet, including her ID and Social Security card.

They found hair dye, suggesting that Puente had changed her appearance.

Police think they've been close to capturing Puente — sometimes trailing her by just hours or even minutes. It's likely she's living under an alias, said Texas Ranger Bobo, "because at this point no traces of Jennifer have been determined."

Before his arrest, Alvarado sometimes saw Moriah when he looked in the mirror, he said. Police said that they had found a towel draped over the dresser mirror in the couple's motel room.

'I had to let go'

Four years later, Ennis police Det. Paul Asby still calls it the worst homicide case of his career. "I just want it closed, I want it finished," he said.

Moriah's mother has been able to find her own sense of peace. It started when she saw Alvarado in court, where she had a chance to say something to him. "I said, 'You have broke us. You have broke my family. But all I want to say to you [is], I will forgive you.'"

"I had to let go of my anger," she explained later. "I had to. I couldn't hold it up. I just couldn't. "

Moriah's cousin, Sabrina Contreras, said it "felt good to have at least one be accountable for what happened to Moriah."

But ultimately police say Puente must bear her share of the responsibility for Moriah's death. Authorities are showing no signs of giving up their search for her.